Daphnia pulicaria. 



541 



a new period of development in July, 1897. From this state- 

 ment of fact it appears that Daphnia pulicaria, as found in 

 lake Mendota, has a biennial period of development extending 

 from July of one year to July of the next, followed by a year in 

 which the species is either absent or its numbers are exceed- 

 ingly small. The study made in July, 1894, seems to have 

 taken the species at the very end of its developmental period 

 after the production of the sexual eggs. One entire cycle was 

 included from July, 1895, to July, 1896, and a second period 

 when the numbers were extremely small, although never en- 

 tirely absent, from July, 1896, to July, 1897. 



Table XVIII. — D. pulicaria. Average number per sq. meter, expressed 



in thousands. 



January 1-15 



January 16-31 



February 1-14.... 

 February 15-28... 



March 1-15 



March 16-31 



April 1-15 



April 16-30 



May 1-15 , 



May 16-31 



June 1-15 , 



June 15-30 



July 1-15 



July 16-31 



August 1-15 



August 16-31 



September 1-15 . . 

 September 16-30 



October 1-15 



October 16-31.. . 

 November 1-15 . . . 

 November 16-30 

 December 1-15.. 

 December 16-31 . 



1894. 



6.4 



8.3 



1.1 



0.8 



a 



a 



a 



a 



a 



a 



a 



a 



1895. 



a 



a 



a 



a 



a 



a 



a 



a 



a 



a 



a 



a 



s 

 11.6 

 19.9 

 38.1 

 33.8 

 98.2 

 26.9 

 23.5 

 49.6 

 58.3 

 141.1 . 

 99.8 



1896. 



88.2 

 24.8 

 64.1 

 43.9 



20.9 



28.0 



118.2 



284.9 



533.6 



168.6 



78.2 



39.3 



11.8 



3.7 



5.9 



23.5 



3.4 



0.4 



a 



s 



s 



8 



S 



a, absent; s, scattering individuals only. 



